In typical integrated planar photonics circuits, such as silicon-on-insulator (SOI) photonic systems, light is confined in a wafer (or chip) plane. In order to couple light in or out of the photonics circuits, the wafer is typically diced into chips and each chip facet is optically polished to allow efficient optical coupling. This optical coupling scheme is referred to as an edge coupling scheme. The input or output light beam in this coupling scheme is parallel to the surface and normal to the edge or facet of the chip.
However, the edge coupling scheme has several limitations. For example, the photonics circuits based on the edge coupling scheme can only be tested and packaged after the wafer is diced up into chips and individual chips are polished. This process for testing and packaging is costly, time consuming, and hardly scalable to high volume manufacturing.
Silicon (Si) photonics is one of the most popular and successful technology platforms based on planar photonics circuits for cost effective opto-electronics integration. Optical waveguide based photonics devices such as lasers, modulators, and detectors are fabricated on silicon-on-insulator (SOI) wafers.
Si waveguides are typically designed with submicron cross-sections, allowing dense integration of active and passive devices to achieve higher speed and lower driving power. Due to the high refractive index contrast between Si and air (or glass), the numerical aperture (NA) of light exiting the Si chip is much larger than the typical NA of optic fibers. As a result, optical mode converters (OMCs) are typically used to improve optical coupling between Si waveguides and optic fibers. Conventional OMC's are based on edge coupling schemes discussed above where light exits along the wafer surface.
One possible way of coupling light out of the wafer surface from a horizontally oriented optical waveguide in the wafer plane is connecting a grating coupler to the waveguide. However, a grating coupler has low efficiency, limited bandwidth (wavelength dependent coupling efficiency), and strong polarization dependence. Typically, light emits at an angle off the surface normal of the wafer, requiring, e.g., an angle-polished fiber to couple light in and/or out of the chip.